use of low cost and abundant metals, such as zinc, at the anode and the absence of hermetic packaging means that the cost of metal–air batteries can be very low. Issues related to dendrite formation at the anode and the absence of a long-life bifunctional (charge and discharge) air cathode have limited the cycle life of zinc–air systems. Solving these two challenges has long been seen as the key to success in these systems. As of 2014, Fluidic Energy is the only company selling commercial rechargeable zinc–air battery systems. Fluidic Energy has been selling commercial backup solutions for telecom sites and in emerging regions since 2011 and has moved into microgrid and other long duration applications. Fluidic has covered a significant number of commercial outages as reported by the company on their website. Description of ARPA-e "MAIL" Program The development efforts within this program were focused on the development of ionic liquids as the electrolyte in metal–air batteries and to overcome some of the known challenges specific to zinc–air batteries. As typically manufactured, such cells are not recharged due to dendrite buildup during the recharging cycle. The dendrites can short circuit the cell when they connect the anode with the cathode. This concept has two significant benefits: Energy density – It is anticipated that this kind of cell can hold over ten times the energy density of a lithium-ion cell, currently the leader for applications requiring rechargeable batteries. Reduced materials costs – Compared to lithium, which is rare, zinc is readily available, and is in fact the fourth most common metal to be mined and used in the world. This could drive the cost of individual cells of roughly the same size and weight to one-third of the cost in mass production. References External links Fluidic Energy website Companies based in Scottsdale, Arizona Metal–air batteries